A different response: A membership program is one for which participants must register in advance (and usually pay a membership fee). The Boy Scouts, for example, is a membership program. If you want to participate, you must become a member.

A non-membership program is one that does not require that you register to participate. It is open to anyone. We need more information in your specific case, but a non-membership program may be available to students on a drop-in basis (they can come anytime; they can come once, or twice, or every day); it may also be free (students don't have to pay to participate).